The present invention relates to electromagnetic and acoustic signatures of objects, more particularly to methods and apparatuses for determining such signatures for complex objects.
Signature reduction of large systems and vehicles is critical to achieving the desired effectiveness of future military systems. As the U.S. Navy progresses towards low observable system designs, new and innovative methods and technologies are needed to meet growing signature reduction needs. The radar or acoustic signature of a body such as a three-dimensional (3D) complex structure can be reduced by shape modification and/or by application of radar or acoustic absorbing material.
Existing numerical methods and computer codes are not adequate or sufficiently accurate for such purposes, since the signature levels have reached a point where second order effects become important. Typically, signature prediction techniques like Physical Optics (PO) or Physical Theory of Diffraction (PTD) are high frequency approximations, and exact methods like Method of Moments (MoM) or Finite Difference Time Domain (FDTD) are computationally intensive and impractical for large objects. Moreover, in many cases, absorbing materials or systems designed to reduce signature are difficult (or impossible) to model accurately using available prediction models, and the only recourse is to use costly full-scale measurements.
In response to U.S. Navy needs to reduce stack and antenna signatures of U.S. Navy ships, Carderock Division of the Naval Surface Warfare Center (NSWCCD) is in the process of developing a low observable (LO) exhaust system with satellite communication (SATCOM) antennae embedded in the associated topside structures. Concept designs for a Low Observable Multi-Function Stack (LMS) are being, developed by the U.S. Navy as part of a FY98 Advanced Technology Demonstration (ATD) program. The present invention is a product or spin-off of the research and development work of the LMS project.
The feasibility of meeting future ship Radar Cross-Section (RCS) signature goals with the LMS was evaluated by the U.S. Navy by performing parametric studies of the LMS shroud shape. The parametric studies showed that the LMS shroud would require radar absorption. A Radar-Absorbing Structural (RAS) material satisfying Radar Cross-Section (RCS) requirements was proposed and developed for the LMS. Bistatic measurements (the accepted method of characterizing the performance of radar absorbing materials) of the proposed LMS material showed that it satisfied the nominal radar attenuation requirements.
A simplified scaled version of the LMS was fabricated using proposed LMS material to evaluate the monostatic radar scattering response. The scaled version of the LMS was a truncated pyramid with approximate dimensions of 6 feet wide by 6 feet long and 3 feet high. The resulting RAS truncated pyramid was measured at the Pt. Mugu radar reflectivity compact range. The RCS measurements of the truncated pyramid showed surprisingly large backscattering from the proposed LMS material.
Attempts to reproduce the RCS measurement results of the truncated pyramid using the measured bistatic absorption of the LMS material as an input to the high frequency Radar Target Signature (RTS) code were not successful. Within the RTS code, the effect of radar absorbing material (RAM) on the radar signature of a scatterer is determined by extracting radar signal attenuation values from a table of measured or calculated bistatic absorption data.
The truncated pyramid or any other target is considered in the RTS code as a collection of basic geometrical shapes, called xe2x80x9cprimitivesxe2x80x9d (such as flat plates, elliptic cylinders, truncated cones, etc.), with the total signature of the object being simply the coherent sum of the signature contributions of each of the individual primitives. The assignment of RAM signal attenuation values to any primitive shape on the model geometry is one of the RTS features. For the assigned material, radar signal attenuation is defined as a specular bistatic response for the appropriate radar frequency, incidence angle, and polarization.
However, some materials and structures (such as the proposed LMS material) have a significant unexpected non-specular scattering with undesired monostatic radar returns. The effect of the non-specular scattering is to dominate what would normally have been very low RCS aspects of the truncated pyramid, thus controlling it""s median RCS. A problem thus presents itself as to how topredict such monostatic non-specular radar returns, and to identify RCS signatures of complex entities such as ship size systems made of such materials and other non-uniform structures.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for rendering signature determinations for complex entities which do not admit of conventional techniques (such as involving computer modeling) for accomplishing such purposes.
It is a further object of the present invention to provide such method and apparatus so as to avoid the necessity of effectuating full-scale measurements of such complex entities.
It is another object of this invention to provide such method and apparatus for rendering signature determinations for complex entities which, due to their material and/or structure, have associated therewith radar cross-section signatures characterized by significant monostatic non-specular radar returns.
The present invention provides a methodology for determining a signature of a complex object. An important benefit of the present invention is that it accounts for non-specular scattering and the accompanying monostatic radar returns.
A notable feature of the present invention, unknown in the art, is the extrapolation of signature information from one object to another object. The inventive methodology uniquely includes an extrapolation of the radar cross-section (RCS) signature (or acoustic signature, for acoustic applications) of a xe2x80x9csamplexe2x80x9d object (such as a scaled-down model of the LMS shroud, a flat RAS panel, or a section of an antenna array) to a full-scale xe2x80x9ccomplexxe2x80x9d object (such as a ship size system) which the sample object represents. Typically according to this invention, the sample object is simpler than is the complex object. According to a principle of the present invention, inasmuch as the present invention""s xe2x80x9cthree-dimensional scattering elementsxe2x80x9d each represent a part of the complex object (e.g., system), the inventive methodology can use either or both of measured sample object signatures and predicted sample object signatures to make extrapolations.
The known methodology for predicting signature data involves (i) taking measured or calculated bistatic signature data from a sample object, and (ii) applying such bistatic signature data to a target object so as to obtain a coherent summation of individual primitives. The present invention provides a new methodology, according to which signature data is extrapolated from a sample object to a complex object (e.g., target). The present invention involves (i) taking measured or calculated monostatic signature data from a sample object, and (ii) extrapolating such monostatic signature data to a complex object so as to obtain an incoherent summation of three-dimensional scattering elements, wherein the three-dimensional scattering elements are reflective of the monostatic signature data. Advantageously, the inventive methodology succeeds in predicting radar-cross section signatures of complex objects which account for monostatic non-specular radar returns from such complex objects; the inventive methodology thus succeeds where the known methodology fails.
According to typical embodiments of this invention, the inventive methodology comprises the actions and rudiments set forth in the following four paragraphs. It is emphasized that the present invention succeeds in estimating either an electromagnetic (e.g., radar) scattering signature or an acoustic scattering signature.
Firstly, the inventive practitioner develops an estimate of the signature (e.g., radar scattering signature or acoustic scattering signature, as the case may be) of a sample object, based on (i) an accurate measurement of the signature of the sample object, or (ii) a high fidelity prediction of the signature of the sample object. According to this invention, the sample object can be any of variously shaped objects, e.g., a plate, a simplified scale model or another shape. The sample object is constructed or composed of the same material as the compound target.
Secondly, based on the estimated signature of the sample object, the inventive practitioner calculates the unit area RCS (for radar scattering signature applications) or the unit area acoustic target strength (for acoustic scattering signature applications) of the sample object as a function of aspect angle and frequency.
Thirdly, the inventive practitioner develops computer geometry of the full size compound target. The inventive practitioner models such geometry using xe2x80x9c3-dimensional (3-D) scattering elements,xe2x80x9d each scattering element representing a specific section (e.g., region or subsystem) of the compound target. The size(s) of the scattering elements will vary depending on the accuracy required, the area of the system, and the shape of the compound target.
Fourthly, using the respective RCS per unit area (for radar scattering signature applications) or acoustic target strength per unit area (for acoustic scattering signature applications) derived from the measured or predicted signature component of the sample object, the inventive practitioner assigns an RCS value (for radar scattering signature applications) or an acoustic target strength value (for acoustic scattering signature applications) in correspondence to each 3-D scattering element used during the estimation of the compound target signature. The RCS estimations (for radar scattering signature applications) or acoustic target strength estimations (for acoustic scattering signature applications) of the compound target use incoherent summation of the 3-D scattering elements as a function of azimuth and frequency.
Accordingly, typical embodiments of the present invention provide a method for determining the radar signature of a target object. The inventive method comprises: (a) rendering a sample object so as to be characterized by the same material as the target object; (b) performing an estimation of the radar signature of the sample object; (c) based on the performing of an estimation, calculating a radar cross-section per-unit-area value for the sample object as a function of aspect angle and frequency; (d) modeling the target object, wherein the modeling includes (i) representing a plurality of three-dimensional elements, and (ii) assigning a per-unit-area signature value to each three-dimensional scattering element; and, (e) performing a summation of the three-dimensional scattering elements as a function of azimuth and frequency. The sample object can have any of diverse shapes, such as a flat plate shape or a scale model shape (i.e., a shape which, usually in simplified form, represents a scale model of the target object).
Generally in accordance with the present invention, the performing of an estimation of the radar signature of a sample object involves measuring and/or predicting. That is, the performing of an estimation of the radar signature includes either or both of: (i) obtaining a measurement of the monostatic backscattering radar cross-section of the sample object; and, (ii) obtaining a high-fidelity prediction of the monostatic backscattering radar cross-section of the sample object. Typically according to this invention, the performing of a summation of the three-dimensional scattering elements as a function of azimuth and frequency includes performing an incoherent summation of the three-dimensional scattering elements as a function of azimuth and frequency.
The present invention admits of practice with respect to various kinds of signatures. In accordance with many inventive embodiments, the inventive method is for extrapolating signature information from a sample object to a target object. The inventive method comprises (a) evaluating the signature per unit area of said sample object as a function of aspect angle and frequency, (b) generating a computer model of said target object, and (c) incoherently summing three-dimensional scattering elements as a function of azimuth and frequency. The computer model represents the target object as including the plural three-dimensional scattering elements. Each three-dimensional scattering element is characterized by the signature cross-section per unit area. According to typical inventive practice, if the signature is an electromagnetic signature, then the signature per unit area is an electromagnetic signature cross-section per unit area; hence, if the electromagnetic signature is a radar signature, then the electromagnetic signature per unit area is a radar cross-section per unit area. If the signature is an acoustic signature, then the signature per unit area is an acoustic target strength per unit area.
The present invention enables accurate and effective signature estimates of a complex system or structure that either does not exist or is difficult to measure, and whose signature is influenced by scattering mechanisms that cannot be effectively modeled analytically. This invention provides a methodology for ascertaining the signature of a full-scale object by extrapolating the radar cross section (RCS) or acoustic signature of a sample object (such as a scaled-down model of a full scale object, a flat panel section of material, or a section of an antenna array) to the full-scale object (such as a ship size system). The inventive methodology can use either/both measured and predicted sample object signatures to make the extrapolations.
The present invention""s methodology allows accurate predictions of electromagnetic or acoustic signatures of compound structures, targets or systems of practically any composition and complexity. It is particularly useful in the areas of low observable (LO) target signatures, where all other analytical methods fail to provide meaningful results.
Among the notable advantages of the present invention""s methodology is low cost. Furthermore, the present invention affords straightforward and accurate signature evaluation of a complex structure or system of any size. Especially valuable is the present invention""s ability to evaluate the effectiveness of signature reduction techniques for future systems without spending precious resources on fabrication and measurement of a full-size target or test system.
Previously known methods and computer models fail to provide accurate results because of inherent approximations (e.g., high frequency codes such as RTS) or computational limitations due to computer memory requirements and processing speed (e.g., Method of Moments codes or Finite Difference Time Domain codes). The present invention was motivated at least in part to overcome these and other shortcomings.
The methodology in accordance with the present invention is being developed and tested by the U.S. Navy for Radar Cross Section (RCS) predictions of the scaled down and ship-size versions of the Low Observable Multifunction Stack (LMS). It is contemplated that the inventive methodology will be used for RCS Signature analyses and reduction, as well as Acoustic Signature analyses and reduction, of military vehicles.